Present passenger side air bag doors are commonly separate parts from the air bag cover portion of the instrument panel and have a skin (also called a "shell") typically made from polyvinyl chloride (PVC) or sprayable aliphatic urethane elastomer with urethane foam as an intermediate layer and a metal or plastic substrate and also made with injection molded polyolefin elastomers that are painted. The separate doors have however created problems regarding proper fit of the doors, color match of the door and in some cases the low and high temperature performance limits of the door. One approach to eliminating such door problems is to form the door integral with the air bag cover skin wherein the door is defined by either a visible or hidden ("invisible") tear seam formed by a weakening groove in the outer or inner side respectively of the cover skin. Such a tear seam is adapted to be readily torn or fractured by the force of an inflating air bag to create a door opening in the cover skin for air bag deployment. An example of such an invisible tear seam is disclosed in U.S. Pat. No. 5,222,760.
Another approach for providing an invisible air bag door tear seam is to form same in the cover skin with a weakened material as disclosed in U.S. Pat. No. 5,288,103. In the latter patent, the cover skin is made by casting thermoplastic material such as PVC in either a spheroidal particle form of the desired size or a cryogenically ground form onto a heated nickel tool surface having the desired shape and appearance side grain (texture). A section of the casting apparatus presses a gasket having the desired tear seam shape (e.g. a H, U or X-shape) against the tool surface during the casting of the cover skin and forms a tear seam defining opening in the cover skin following fusing thereof and removal of the gasket. The tear seam defining opening in the cover skin is then filled with a lesser strength thermoplastic material such as glass or carbonate filled PVC while the cover skin remains on the heated mold surface to thus form in place an integrally bonded tear seam strip in the cover skin. This produces an invisible tear seam if the filler used and the processing conditions are such that the tear strip that is formed in place on the tool surface has a perfect or near perfect porosity and color match with the cover skin. If not, the cover skin with integral tear seam strip can be painted to provide this effect.
Certain automobile manufactures have prescribed that the elastomer used to make an air bag cover skin with an integral tear seam be capable of providing extremely low temperature performance as well as being capable of meeting certain requirements for withstanding prolonged levels of artificial weathering which simulates exposure to the heat and ultra violet radiation and sunlight. With respect to low temperature performance, it is desired that the cover skin including the tear seam not suffer cold embrittlement and then be prone to fragment or shatter into small pieces when the seam is torn for air bag deployment at a very low temperature. For example, the PVC elastomers presently used are capable of meeting the basic requirements for weathering and providing satisfactory performance with regard to an integral tear seam in the temperature range of about -20.degree. C. to 80.degree. C. but not down to -40.degree. C. or to as high as 100.degree. C. which is now being specified by some automobile manufacturers. A further more specific example is where the cover skin is made with cast PVC particles or powder and the tear seam is made with filled PVC as in the above U.S. Pat. No. 5,288,103 wherein the PVC for the tear seam is filled with 40% by weight water ground calcium carbonate having a mean particle size of 5.5 microns and a particle size distribution that ranges from 0.5 to 50 microns. It was found that the resulting cover skin and formed in place tear seam does provide satisfactory performance with respect to air bag deployment in the temperature range of -17.degree. C. to 76.degree. C. which is generally acceptable by automobile manufactures except for those now requiring a far more extended performance range of -40.degree. C. to 100.degree. C.
On the other hand, there is now known to be a light stable aliphatic thermoplastic urethane elastomer that in addition to meeting the basic requirements of resistance to heat, ultra violet radiation and sunlight, also has the ability to provide satisfactory performance with respect to an integral tear seam strip formed with the same urethane elastomer in the desired temperature range of -40.degree. C. to 100.degree. C.; this particular urethane elastomer exhibiting a glass transition temperature of -55.degree. C. and being disclosed in U.S. patent application Ser. No. 319,614 filed Oct. 7, 1994 entitled "LIGHT STABLE ALIPHATIC THERMOPLASTIC URETHANE ELASTOMERS AND METHOD OF MAKING SAME" and assigned to the assignee of this invention and which is hereby incorporated by reference. However, a suitable weakened or lesser strength thermoplastic material based on such an aliphatic urethane elastomer is not presently known to be available for making an air bag door tear seam that is compatible with conventional casting processing methods for the cover skin.